Method of making fluorinated polyolefin laminates



United States Patent" FLUORINATED METHOD OF MAKIN POLYOLEFIN LAMINATEES Walter H. Smarook, Somerville, N.J., assignor to UHlOll Carbide Corporation, a corporation of New York No Drawing. Continuation-impart of application Ser. No. 458,775, May 25, 1965. This application Oct. 23, 1965, S No. 504 241 er 16 Claims. (Cl. 156-151) ABSTRACT OF THE DISCLOSURE Polyvinyl fluoride, polyvinylidene fluoride, polychlorotrifluoroethylene and polytetrafluoroethylene can be bonded to electroconductive anodic substrate onto which a carboxyl containing a-olefin polymer has been electrodeposited from an aqueous bath containing an admixture of water and a salt of a carboxyl contaming ocolefin polymer having monovalent cations, at temperatures of about 180 C. to 260 C.

This is a continuation-in-part of application Ser. No. 458,775, filed on May 25, 1965.

This invention relates to laminates of fluorinated polyolefins and more particularly to those made w th electroconductive substrates coated with electrodepositcd carboxyl-containing a-olefin polymers.

Fluorinated polyolefins exhibit several unique properties including resistance to weathering, outstanding mechanical properties and inertness towards a wide variety of chemicals, solvents and staining agents. Four of these fluorinated polyolefins, viz., polyvinyl fluoride, polyvinylidene fluoride, polychlorotrifluoroethylene and polytetrafiuoroethylene are particularly worthy of note and have been used to protect various substrate surfaces by forming laminates with them. To obtain strong bonds between these substrates and these fluorinated polyolefins a bonding agent must be used. To date, the agents available for this purpose have not been wholly satisfactory.

It has now been discovered that electroconductive substrates can be readily bonded to fluorinated polyolefins having up to 4 fluorine atoms per each olefin repeating unit by a method which comprises: I

(a) electrodepositing a carboxyl containing wOIEfill polymer onto an electroconductive anodic substrate from an aqueous bath containing an admixture of water and a salt of a carboXyl-containing a-olefin polymer havmg monovalent cations, the a-olefin content of said polymer salt being at least 50 mole percent of the total polymer salt;

(b) surface treating a fluorinated polyolefin having up to 4 fluorine atoms per each olefin repeating unit;

(c) contacting the substrate having electrodeposited carboXyl-containing a-olefin polymer thereon with the surface treated fluorinated polyolefin; and

(d) applying a pressure of at least 1 p.s.1.g. at a temperature of about 180 C. to 260 C. for at least 0.01 second.

The laminates thus prepared are useful for the fabrication of weather-resistant and solvent-resistant load bearing and decorative structures incorporating the physical properties of the substrate with the chemical inertness of the fluorinated polyolefin.

Although not essential, it is preferred to employ as the carboxyl containing u-olefin polymers of this invention interpolymers of a-olefins having the general formula:

where R is selected from the group consisting of hydrogen and alkyl radicals having up to carbon atoms,

the olefin content of said interpolymer being at least 50 mole percent of the total interpolymer and interpolymerized therewith an a,B-ethylenically unsaturated carboxylic acid having one or more carboxyl groups, said unsaturated carboXylic acid constituting up to about 50 mole percent of the total interpolymer.

However, the present invention is not limited to interpolymers derived from the interpolymerization of an aolefin and an u,/3-ethylenically unsaturated carboxylic acid. The starting polymer used to make the polymer salts used in this invention can also be provided by oxidizing olefinic polymers, such as those described in US. Patent 3,155,644, by grafting carboxylic acid containing monomers onto an olefin polymer backbone by methods well known in the graft polymerization art such as the method described in US. Patent 2,970,129 which are incorporated herein by reference or by grafting monomers such as carboxylic acid derivatives, i.e., esters, anhydrides, amides, nitriles and the like onto an olefin polymer backbone followed by conversion to free carboxylic acid group after grafting.

Also included within the purview of this invention are halogenated, carboxyl containing a-olefin polymers. The method of introducing the halogen into these polymers is not at all critical and so can be accomplished by first preparing a halogen free interpolymer of, for example, ethylene-acrylic acid and then halogenation that interpolymer by methods well known in the art or by interpolymerizing a halogen containing vinyl monomer with an wolefin and an unsaturated carboxylic acid. A specific example of this latter class of interpolymers is one obtained by interpolymerizing ethylene, vinyl chloride and acrylic acid. Other examples include interpolymers of ethylene-vinylidene chloride-acrylic acid, ethylene-vinyl chloride-methacrylic acid, ethylene-vinylidene chloridemethacrylic acid, ethylene-vinyl bromide-acrylic acid, ethylene-vinyl bromide-methacrylic acid, ethylene-vinyl fluoride-acrylic acid, ethylene-vinyl fluoride-methacrylic acid, ethylene-vinylidene fluoride-acrylic acid, ethylenevinylidene fluoride methacrylic acid, ethylene vinyl iodide acrylic acid, propylene vinyl chloride acrylic acid, propylene vinyl chloride methacrylic acid, propylene vinylidene chloride acrylic acid, propylenevinylidene chloride methacrylic acid and the like.

As indicated above, the a-olefins preferably employed in the polymers of this invention are a-olefins having the general formula:

RCH=CH where R is either a hydrogen or an alkyl radical having up to 10 carbon atoms. Thus, suitable a-olefins include, ethylene, propylene, butene 1, pentene 1, hexene l, neohexene, octene l, nonene l, decene 1,3 methylbutene l, 4 methylpentene l, 3 methylhexene 1, 4,4-dimethylheXene-l and the like. Although polymers of higher olefins can be used, they are not as commercially available or economical as the lower olefins.

The a,fl-ethylenically unsaturated carboxylic acids used in the polymers of this invention preferably have 3 to 8 carbon atoms, although those having a greater number of carbon atoms can also be used, if desired. Specific examples include: acrylic acid, methacrylic acid, ethacrylic acid, itaconic acid, maleic acid, fumaric acid, and half esters of the above dicarboxylic acids such as, methyl hydrogen maleate, methyl hydrogen fumarate, ethyl hydrogen maleate, ethyl hydrogen fumarate and the like.

The starting polymers used to make the polymer salts of this invention are not limited to two components. Therefore, additional copolymerizable monomers can be employed together with the olefin and carboxylic acid comonomers. The scope of the starting polymers which can be used is exemplified, although not limited by the following interpolymers: ethylene/acrylic acid interpoly-mers, ethylene methacrylic acid interpolymers, ethylene/itaconic acid interpolymers, ethylene/ methyl hydrogen maleate interpolymers, ethylene/maleic acid interpolymers, ethylene/ acrylic acid/methyl acrylate interpolymers, ethylene/acrylic acid/ethyl acrylate interpolymers, ethylene/methacrylic acid/ methyl methacrylate interpolymers, ethylene/methacrylic acid/ethyl methacrylate interpolymers, ethylene/ acrylic acid/methacrylic acrylic acid interpolymers, ethylene/methacrylic acid/ methyl acrylate interpolymers, ethylene/ acrylic acid/ methyl methacrylate interpolymers, ethylene/methyl hydrogen maleate/ethyl acrylate interpolymers, ethylene/ acrylic acid/vinyl acetate, ethylene/methacrylic acid/ vinyl acetate interpolymers, ethylene/ propylene/ acrylic acid interpolymers, ethylene/propylene/methacrylic acid interpolymers, ethylene/'maleic acid/ethyl vinyl ether interpolymers, ethylene/butene-l/acrylic acid interpolymers, ethylene/neohexene/ acrylic acid interpolymers, propylene/ acrylic acid interpolymers, butene-l/ acrylic acid interpolymers and the like.

The polymer salts used in this invention need not be limited to 2 components, i.e., one a-olefin and one unsaturated carboxylic acid salt moiety. Even when the starting polymer is composed of only 2 components, viz., one a-olefin and one unsaturated carboxylic acid, the resultant salt can have 3 or more components. To cite a concrete case in point, an ethylene/acrylic acid interpolymer can be partly neutralized with sodium hydroxide to afford an interpolymer salt consisting of 3 components, viz., ethylene/acrylic acid, and sodium acrylate mers. It will be readily apparent to those skilled in the art that polymer salts containing innumerable combinations of a-olefins and unsaturated carboxylic acids, salts and acid derivatives are contemplated within the purview of this invention.

The carboxyl content of the starting olefin polymer is preferably in the range of about 5 to 25 mole percent of the total polymer although contents of about 1 to 50 mole percent can also be employed, if desired. The starting olefin polymer used to make polymer carboxyl salts in this invention should contain sufficient carboxyl groups or permit the formation of at least water-dispersible salts and preferably Water-soluble salts. Solubility increases as the carboxyl salt moiety content of the polymer is increased.

The carboxyl salt moiety content of these polymer salts is preferably about 3 to 25 mole percent of the total polymer salt although other ranges can also be employed, if desired. The term carboxyl salt moiety includes both the carboxyl anion and the monovalent cation.

The preferred monovalent cations in the polymer salts of this invention are Na K+, Li+ and wherein each of R R and R is hydrogen or an organic radical, containing up to carbon atoms, which is unsubstituted or substituted with a hydrophilic group such as hydroxyl, amino, imino or cyclic ether groups and wherein two of the organic radicals when taken together form a heterocyclic compound.

The organic radical can be a hydrocarbon radical such as an alkyl, cycloalkyl, aryl, alkaryl or aralkyl radical. Suitable alkyl radicals include methyl, ethyl and isopropyl radicals. Representative cycloalkyl radicals include cyclobutyl, cyclopentyl and cyclohexyl radicals. Representative aryl radicals include phenyl and naphthyl radicals. Among the alkaryl and aralkyl radicals which can be used are benzyl, cumyl, tolyl and Xylyl radicals.

Representative substituted hydrocarbon radicals include: ethanolamine, diethanolamine, triethanolamine,

ethylene diamine, diethylene triamine, triethylenetetramine, dimethylamino propylamine, diethylamino propylamine, N-(hydroxyethyl)ethylene diamine, N-(Z-hydroxypropyl) ethylene diamine and the like.

Representative heterocyclic compounds include piperazine, 2,5-dimethyl piperazine, piperidine, morpholine and the like.

These polymer carboxyl salts can be prepared by neutralizing the starting carboxyl containing polymer with metal salts, such as carbonates or bicarbonates, metal bases such as hydroxides or alkoxides, amine bases such as trimethyl ammonium hydroxide, monomethyltriethyl ammonium hydroxide, dimethylphenyl ammonium hydroxide and the like, aliphatic amines such as ethanolamine, ethylene diamine, N-(hydroxyethyl)ethylene diamine and the like, cyclic amines such as piperazine, pyridine, piperidine, morpholine and the like, metal alkyls such as sodium ethyl, butyl lithium and the like, metal aryls such as phenyl lithium, potassium naphthalene and the like, hydrides of sodium, potassium or lithium, amides of sodium or potassium, oxides such as sodium peroxide or in the case of alkali metal salts even With the free alkali metal itself. Preferred bases are alkyl ammonium hydroxides, ammonium hydroxide, lithium hydroxide, sodium hydroxide and potassium hydroxide. Any method known in the art can be used to effect this neutralization process.

The concentration of polymer salt in the Water-salt bath is not narrowly critical. A preferred range is about 5.0 to 40% solids by weight with about 5% to 20% being particularly preferred.

Polyvinyl fluoride, polyvinylidene fluoride, polychlorotrifluoroethylene and polytetrafiuoroethylene can be prepared by methods well known in the art and described by C. E. Schildknecht on pp. 442-4, 472476, and 484 respectively of Vinyl and Felated Monomers, John Wiley and Sons, Inc., New York, 1952.

The bonding of the coated electroconductive substrate to the fluorinated polyolefin can be easily effected at moderate pressures, e.g., 1 p.s.i.g. and above, and temperatures of about 180 C. to 220 C. The time required to achieve a satisfactory bond is not critical so that any time from about 0.04 second and about can be used. Any conventional press equipment known in the art can be used in both a batch or continuous bonding operation.

Electrodeposition temperature over the range of about 0 C. to about C. is not critical as regards the chemical nature of the electrodeposited interpolymer. Temperature does, however, have a quantitative effect as to the amount of interpolymer deposited on an anodic substrate. This effect varies somewhat with each substrate but in general, there is a minimum amount deposited at about 40 C. with greater amounts deposited above or below this temperature. For reasons of convenience and economy, temperatures of about 20 C. to 30 C. are preferred although any temperatures Within the freezing point and boiling point of the bath used can be employed.

Electrodeposition time is not critical since the process is inherently self-regulating. Thus, current flows in general until a continuous insulating coating is deposited on the anodic substrate. When the resistance of this deposited coating exceeds a certain value the current flow essentially ceases. In most cases this point is reached in a fraction of a second as determined with an oscilloscope-camera combination, the oscilloscope having a dual trace feature which allows the simultaneous recording of curves of voltage vs. time and current vs. time during any one deposition. Highly porous anodic substrates will conduct current for longer periods because of the time required for diffusion to conducting surfaces.

Any available source of direct current can be used in the practice of the invention such as a motor-generator unit, batteries, rectified alternating current and the like. It was found convenient to employ direct current obtained by the full-wave rectification of a variable autotransformer controlled 115 volt alternating current, filtered to low residual ripple by a well-known impedance-capacitance arrangements. It is preferred to use a direct current of at least 5 volts D.C. There is no maximum voltage other than that imposed by the heat generated in the salt bath and the efiiciency of heat transfer of the system used.

The anodic substrates which can be used in this invention can be metallic or non-metallic as long as the surface is electroconductive. Particularly useful metallic substrates include aluminum, copper, steel, nickel, chromium, mercury and alloys of these as well as tin plate, galvanized screen and the like, as well as metal coated non-conductors such as thin metal coated plastic articles obtained by sputtering, sintering, evaporating, dip coating and other methods well known in the art. Non-metallic conductive substrates including carbon, graphite, graphite cloth and the like, can easily be employed as well as non-conductors coated with non-metallic conducting substrates,

The cathode should, of course, also be electroconductive and its composition is not critical although it preferably should be inert under the reaction conditions used in the electrodeposition process. Several materials which are suitable include lead, stainless steel, carbon, graphite, tin, platinum, rhodium, and the like. The cathode may be used as a simple electrode immersed in the water-salt bath or may be constructed to serve as the cell to contain the bath, or may be in electrical contact with the bath via an ionically permeable membrane, e.g., salt bridge porous plate combination or ionically permeable membrane, without literally touching the bath.

It is essential to pre-treat the surface of the polyfluoroethylenes prior to its being bonded to a particular coated substrate preferably by exposure to ozone, nitrous oxide, ultra-violet light, gamma irradiation, sodium in liquid ammonia, sodium with naphthalene in tetrahydrofuran and like methods. It is particularly preferred to use sodium in liquid ammonia or sodium with naphthalene in tetrahydrofuran.

The invention is further described by the examples which follow in which all parts and percentages are by weight unless otherwise specified.

Examples ll0.-Electrodeposition of carboxyl containing a-olefin polymer onto electroconductive surfaces Panels of aluminum (2024-TS 3) 3" x 5" x 0.005 and cold rolled steel 3 x 5" X 0.011" were cleaned and de greased successively with acetone and methylene chloride. In separate experiments the aluminum panels and then the steel panels were immersed in a 20% solids aqueous solution of a 91.88/0.8/7.4 mole percent ethylene/acrylic acid/ sodium acrylate terpolymer by suspending them from a clamp attached to the positive lead of a DC. power supply, The terpolymer solution was confined in a tin plate beaker insulated from the surrounding apparatus and connected to the negative power supply lead. Direct current was obtained by the full-wave rectification of a variable auto-transformer controlled 115 volt alternating current, filtered to a low residual ripple by known impedance-capacitance arrangements. The power supply was pre-set to the desired voltage and a switch closing the circuit thrown. In general, a potential of 75-l00 volts D.C. was applied across the substrate anode and beaker cathode for about one minute although carboxyl containing wolefin polymer was deposited in less than one second for most substrates. The substrate coated with electrodeposited polymer was then removed from the polymer salt solution, washed with a stream of water and dried at 80-120 C. for about 30 minutes. The thickness of the electrodeposited polymer coating ranges from about -20 microns.

Tin, zinc, chromium, copper, nickel, brass, carbon and graphite are also coated with electrodeposited carboxyl containing a-olefin polymer in the manner described above.

Examples 11-20. Bonding of conductive substrates coated with electrodepositedcarboxyl containing a-olefin polymer to polyvinyl fluoride Aluminum panels prepared as in Example 1 coated by electrodepositing an ethylene/acrylic acid/sodium acrylate terpolymer thereon were placed face to face with 4" x 6" x 0.002" sheets of polyvinyl fluoride previously treated by immersion in a mixture of sodium (0.4 g.) in anhydrous liquid ammonia g.), between the steam heated platens of a hydraulic press. A pressure of l400 p.s.i.g. at l70-200 C. was applied for about one minute. This simple technique bonded the coated aluminum panel securely to the sheets of polyvinyl fluoride affording laminates with excellent peel strength. A load of 1365 g. per inch was required to peel the film from the laminate. This operation was repeated with the cold rolled steel panels prepared with coatings as in Example 2 and also afforded laminates of coated steel panels with polyvinyl fluoride having peel strengths of about 3.0 lbs/inch. As controls uncoated aluminum and cold rolled steel panels were placed in the hydraulic press with sheets of polyvinyl fluoride. No bonding of these components resulted.

Laminates of polyvinyl fluoride with tin, zinc, chro- -rnium, copper, nickel, brass, carbon and graphite coated with electrodeposited canboxyl containing tat-olefin polymer as described in Examples 3-10 can be obtained in a similar manner.

Attempts to laminate aluminum or cold rolled steel with a layer of ethylene/acrylic acid copolymer and polyvinyl fluoride sheet were unsuccessful.

Examples 2130.B0nding of conductive substrates coated with electrodeposited carboxyl containing a-olefin polyvinyl fluoride sheets were unsuccessful.

Aluminum and cold rolled steel panels coated by electrodepositing an ethylene/acrylic acid/sodium acrylate terpolymer as in Examples 1 and 2 were bonded to sheets of polyvinylidene fluoride 4" x 6 x 0.032" by the bonding technique described in Examples 11 and 12. Laminates of both metals having peel strengths of about 4.14 lbs. per inch were obtained. Uncoated controls did not bond to the vinylidene fluoride sheets.

Laminates of polyvinylidene fluoride, previously im-' mersed in a mixture of sodium (0.4 g.) and liquid am- \monia (100 g.), with tin, zinc, chromium, copper, nickel, brass, carbon and graphite coated with electrodeposited carboxyl-containing u-olefin polymer as described in Examples 3-10 can be obtained in a similar manner.

Attempts to laminate aluminum or cold rolled steel with a layer of ethylene/acrylic acid copolymer and a sheet of polyviny-lidene fluoride were unsuccessful.

Examples 31-40 Examples 41-42 The general procedures described in Examples 1-10 and 11-20 were used with aluminum panels 5 mils thick to prepare polyvinyl fluoride-aluminum laminates at 220 C. under a pressure of about 40 p.s.i. In these experiments the dwell or contact time that is, the time interval during which the bonding operation was carried out, was lowered '7 to about 4-5 seconds. By raising the bonding temperature to 240-260 C. contact times of about 0.04 to 0.1 second are suflicient to effect excellent adhesion between aluminum and polyvinyl fluoride.

Continuous in-line bonding of polyvinyl fluoride to aluminum can be carried out by moving sheets of aluminum and polyvinyl fluoride continuously in contact with one another through a heated nip roll.

Examples 43-54 Cold rolled steel panels 4" x 6" x 0.011 and aluminum panels 4 x 6" x were laminated with polyvinyl fluoride following the procedures of Examples 1-10 and 11-20. The resultant laminates were then subjected to various metal forming operations as a practical empirical test of adhesion. The laminates were subjected to inside bends of 15, and 90 and outside bends of 360. The laminates were also subjected to indentation or dimple tests by impinging a /2 diameter steel ball mounted in an arbor press on laminates placed over a female die. In all tests there was no delamination observed.

Examples -68 When Examples 41-54 are repeated using polyvinylidene fluoride in place of polyvinyl fluoride similar re- Aluminum and cold rolled steel panels coated by electrodepositing an ethylene/acrylic acid/ sodium acrylate terpolymer thereon as in Examples 1 and 2 were bonded to sheets of polychlorotrifluoroethylene 2" x 4" x 0.071 to 0.083" by the bonding technique described in Ex amples 11 and 12. Laminates of both metals having peel strengths of about 4.5 lbs. per square inch were obtained. Uncoated lmetal controls did not bond to the polychlorotrifluoroethylene sheets.

Laminates of polychlorotrifluoroethylene with tin, zinc, chromium, copper, nickel, brass, carbon and graphite coated with electrodeposited carboxyl containing 1xolefin polymers by the technique described in Examples 3-10 can be obtained in a similar manner having comparable peel strengths.

Examples 79-88 Aluminum and cold rolled steel panels coated by electrodepositing an ethylene/acrylic acid/sodium acrylate tenpolymer thereon as in Examples 1 and 2 were bonded to sheets of polytetrafluoroethylene 2 x 4" x 0.005 to 0.020" by the bonding technique described in Examples 11 and 12. Laminates of both metals having peel strengths of about 4.5 lbs. per square inch were obtained. Uncoated metal controls did not bond to the polytetrafluoroethylene sheets.

Laminates of polytetrafluoroethylene with tin, zinc, chromium, copper, nickel, brass, carbon, and graphite coated with electrodeposited carboxyl containing a-olefin polymers by the technique described in Examples 3-10 can be obtained in a similar manner having comparable peel strengths.

Examples 89-98 When Examples 41-54 are repeated using polychlorotrifluoroethylene in place of polyvinyl fluoride similar results are obtained.

Examples 99-108 When Examples 41-54 are repeated using polytetrafluoroethylene in place of polyvinyl fluoride similar results are obtained.

Although the invention has been described in its preferred forms with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes can be made without departing from the spirit and scope of the invention.

What is claimed is:

8 1. Methodof bonding electroconductive substrates to fluorinated polyolefins having from 1 to 4 fluorine atoms per each olefin repeating unit which comprises; v

(a) electrodepositing a carboxyl containinga-olefin polymer onto an electroconductive anodic substrate from an aqueous bath containing. an admixture of water and a salt of a carboxyl containing a-olefin polymer having monovalent cations, the afolefin con.- tent of said polymer salt being at least 50 mole percent of the total polymer salt; v (b) surface treating a, fluorinated polyolefin having up to 4 fluorine atoms per each olefin repeating unit; (c) contacting the substrate. having electrodeposited carboxyl-containing tat-olefin, polymer thereon with the surface treated fluorinated polyolefin; and

(d) applying a pressure of at, least one p.s.i.g. at a temperature of C. to 260 C. for atleast 0.01 second. 2. Method claimed in claim 1 ethylene.

3. Method claimed inclaiml wherein the carboxyl-containing wolefinpolyrner salt contains upto 50. mole per eent of an /B-ethylenically unsaturated carboxylic acid having at least one carboxyl group. p

4. Method claimed in claim 3 wherein thecthylenically unsaturated carboxylic acid is acrylic acid and the acrylic acid anions. plus the cations comprise about 10 to 25 mol percent of. the total olefin polymer salt.

5. Method claimed in claim 1 wherein the monovalen cations are Nat cations. 6. Method, claimed in claim 1 wherein the monovalen cations are K+ cations.

7. Method claimed in claim 1 wherein the monovalent cations are wherein the wolefin is wherein each of R R and R is selected from the group consisting of hydrogen and. substituted and unsubstituted organic radicals containing up to 10'carbonatoms, Whereinsaid substituents are selected from the group consisting of hydroxyl, amino, imino and cyclic ether groups with the proviso-that two of the organic radicals can be combined to form a heterocyclic compound.

8. Method claimed in claim 1 wherein the fluorinated polyolefin is polyvinyl fluoride.

9. Method claimed in claim 1 wherein the fluorinated polyolefin is polyvinylidene fluoride.

10. -Method claimed in claim 1 wherein the fluorinated polyolefin is polychlorotrifluoroethylene.

11. Method claimed in claim 1 wherein the fluorinated polyolefin is polytetrafluoroethylene.

12. Method claimed in claim 1 wherein the electroconductive substrate is a metal.

13. Method claimed in claim 10 wherein the metal is aluminum.

14. Method claimed in claim 10 wherein the metal is cold rolled steel.

15. Method claimed in claim 1 wherein the electrodeposition is carried out at a potential of at least 5 volts DC.

16. Method claimed in claim 1 wherein the surface treatment of the fluorinated polyolefin is effected'in a mixture of sodium in liquid ammonia.

References Cited- UNITED STATES PATENTS 2,705,691 4/1955 Panagrossi et a1 161-165 2,774,704 12/1956 Smith 161-189 2,809,130 10/1957 Rappaport 161--189 X 2,834,763 5/1958 Halpern et al. 161l89 X 2,871,144 1/1959 Doban 161l89 X 2,941,912 6/1960 CoX et a1. 16l189 X HAROLD AN SHER, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,410,740 November 12, 1968 Walter H. Sm arook It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, lines 35 to 38, the formula should appear as she below:

l Z R Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

